java Map刪除意外行為

Question

碼：

public class FeatureBuilder implements IFeatureBuilder {

   private final Map< java.awt.Shape, ShapeAttributes > shapes = new HashMap<>();

   [...]

   @Override
   public void addToContainer( java.awt.Shape shape ) {
      System.err.println( shape.getClass());
      shapes.put( shape, new ShapeAttributes( ... ));
   }

   @Override
   public void removeFromContainer( double x, double y ) {
      final Set< Shape > rm  = new HashSet<>();
      final Point2D      loc = new Point2D.Double( x, y );
      for( final Shape shape : shapes.keySet()) {
         if( shape.contains( loc )) {
            System.err.println( "shapes.containsKey( shape ): " +
               shapes.containsKey( shape ));
            rm.add( shape );
         }
      }
      System.err.println( "cardinality before removal : " + shapes.size());
      shapes.keySet().removeAll( rm );
      System.err.println( "cardinality after  removal : " + shapes.size());
   }

輸出：

class java.awt.geom.Rectangle2D$Double
shapes.containsKey( shape ): false      <<<<<<< This is unexpected!
cardinality before removal : 1
rm cardinality             : 1
cardinality after  removal : 1

我很驚訝：在Map.keySet()上由for迭代器檢索的實例不是Map的鍵！

這怎么可能？

此方法的主要缺陷是放置在rm中的Shape的選定實例不會從shapes刪除。

閱讀完答案后，代碼變為：

public class DecoratedShape {

   public final Shape _shape;
   public /* */ Color _stroke      = Color.BLACK;
   public /* */ float _strokeWidth = 3.0f;
   public /* */ Color _fill        = Color.BLACK;

   public DecoratedShape( Shape shape, Color stroke, float strokeWidth, Color fill ) {
      _shape       = shape;
      _stroke      = stroke;
      _strokeWidth = strokeWidth;
      _fill        = fill;
   }

   public boolean contains( Point2D loc ) {
      return _shape.contains( loc );
   }

   public void paint( Graphics2D g ) {
      g.setStroke( new BasicStroke( _strokeWidth ));
      g.setColor( _fill );
      g.fill( _shape );
      g.setColor( _stroke );
      g.draw( _shape );
   }
}

public class FeatureBuilder implements IFeatureBuilder {

   private final List< DecoratedShape > _shapes = new LinkedList<>();
   [...]

   @Override
   public void addToContainer( Object o ) {
      _shapes.add( new DecoratedShape((Shape)o, _stroke, _strokeWidth, _fill ));
   }

   @Override
   public void removeFromContainer( double x, double y ) {
      final Set<DecoratedShape> rm = new HashSet<>();
      final Point2D loc = new Point2D.Double( x, y );
      for( final DecoratedShape shape : _shapes ){
         if( shape.contains( loc ) ){
            rm.add( shape );
         }
      }
      _shapes.removeAll( rm );
   }

   public void paint( Graphics2D g ) {
      for( final DecoratedShape shape : _shapes ) {
         shape.paint( g );
      }
   }
}

現在，它按預期工作......非常感謝！

Answer 1

跟進@ RohitJain的評論，這是一個重現行為的簡單示例：

Map<Shape, Object> map = new HashMap<>();
java.awt.geom.Rectangle2D.Double shape = new java.awt.geom.Rectangle2D.Double(1, 1, 1, 1);
map.put(shape, null);
System.out.println(map.size()); //1
shape.setRect(2, 2, 2, 2); //mutate
System.out.println(map.size()); //still 1
map.keySet().remove(shape);
System.out.println(map.size()); //still 1

根本問題是Shape在地圖中發生了變異。

Answer 2

這並不能解釋為什么您的代碼不起作用，但它會解決您的問題並使方法更有效，因為沒有分配臨時集：

public void removeFromContainer(double x, double y) {
    final Point2D loc = new Point2D.Double(x, y);
    Iterator<Shape> iter = shapes.keySet().iterator();
    while (iter.hasNext()) {
        if (iter.next().contains(loc))
            iter.remove();
    }
}

編輯：我懷疑你在將它們添加到地圖后修改了形狀，因此檢查containsKey時的哈希碼與將Shape添加到地圖時使用的哈希碼不匹配。